Nikon F3 with DW-3. Enough said.
Or Nikon F2 with DW-1.
Or Nikon F with DW-1.
If you need autofocus and matrix metering, Nikon F4 with DW-20.
You could shoot any of these just by taking the prism off, in a pinch.
These are just the Nikons, but most of the other big manufacturers offered a top tier camera with a removable prism and waist level capability.
No need to reinvent the wheel here. A shoe mounted waist level finder already exists but if you find one, I want it. They were made for Bell & Howell Filmo 70 motion picture cameras. It's hard to even find a photo of one, let alone a real sample. I've never seen one in real life. A few defense contractors made accessory waist and eye level finders with projected framelines for a normal lens (or whatever lens the camera was spec'd with) but they are BIG and not at all unobtrusive, which is what I think you're going for here.
The problem with this idea past the recommendation of the mirrored viewfinder of an old folding camera is brightness and eye relief. Assuming you actually want to use it about a foot away, and not with your eyeball right in the thing (because eye-level angle finders are available for every eyepiece of every system) you are going to need a big screen that is bright. At this point, it's easier to just give in and get a Nikon F3 with DW-3, enjoy excellent metering, one of the best cameras ever made, and a pretty good waist level finder.
I love carrying my FA for quick action in changing lighting, especially if I'm wandering around in and out of deep shadows and bright highlights. If I'm going to be laying on the ground taking photos of bugs or flowers or whatever, I'm grabbing my F2 and sticking the prism in my bag.
Or get a TLR and call it a day.
It's mostly a solution to a problem that doesn't really exist to the extent that a workaround is necessary.
Phil Forrest